Democrats outnumber Republicans 6-to-1 in Gotham. So why would anyone fight for the GOP nomination for mayor?

Two reasons: First, New Yorkers haven’t elected a mayor running on the Democratic line since David Dinkins in 1989 (and we know how that turned out). By contrast, two of the city’s last three substantive mayors — Rudy Giuliani and Michael Bloomberg — won as Republicans.

That’s worth keeping in mind as we survey today’s GOP field. The list of contenders now includes businessman John Catsimatidis, nonprofit founder George McDonald, publisher Tom Allon, former Bronx Borough President Adolfo Carrion and former Metropolitan Transportation Authority boss Joe Lhota.

Until recently, the first four names were Democrats. But these are Democrats with sane positions on public-employee unions.

In a comment that might baffle Topeka but makes perfect sense in the Big Apple, Catsimatidis explained his purpose in seeking the Republican slot on the ballot: “I want to run as a pro-business Democrat.”

That points to the other irony of this year’s GOP primary: The only one who seems to have always been a Republican is Joe Lhota.

Though Lhota has never before run for office, he comes off a successful stint at one of the city’s most hated public entities (the MTA), served as deputy mayor under Giuliani and understands that the government-employee unions have way too much power.

In addition, his private-sector experience gives him something distinctly lacking in this city’s politics: an appreciation that the standard response to worries about affordability — larger public subsidies — isn’t going to keep New York competitive.

So far, the Democrats have limited their “boldness” to saying which achievements of their predecessors (low crime, mayoral control of schools, etc.) they’re willing to maintain. The Republican challenge is to force the debate into how New York should move to the next level of reform.